You can set a negative inset in the construction settings for your cabinet. I do this, for example when an upper gets under cabinet lighting and a valence, at the end of a run to hide the lighting and accommodate the height of the valence.

I understand what your saying but what I'm trying to do is have a set of angled cabinets that follow the angle of the ceiling from the peak to the wall while keeping the bottoms at 54" from the floor. In essence every cabinet that gets hung on the wall consistently grows like a triangle at the top.